JMJD3/HDAC-IN-1 (compound A5b) is a dual inhibitor targeting Jumonji domain-containing protein demethylase 3 (JMJD3) and histone deacetylase ( HDAC1 , IC 50 =16 nM). JMJD3/HDAC-IN-1 promotes hypermethylation of histone H3K27 and hyperacetylation of H3K9, and also cleaves caspase-7 and PARP to induce apoptosis. JMJD3/HDAC-IN-1 effectively inhibits cancer cell cloning, migration, and invasion.